In the field of infrared detectors, there is a known technique of using devices arranged in a matrix form that are able to operate at ambient temperature, in other words they do not need to be cooled to very low temperatures, unlike the detection devices known as “quantum detectors”, which need to operate at a very low temperature, typically that of liquid nitrogen.
These non-cooled detectors conventionally use the variation of a physical variable of an appropriate material, as a function of the temperature, in the vicinity of 300K. In the case of bolometric detectors, this physical variable is the electrical resistivity of the material.
A non-cooled detector of this type generally associates:                means for absorbing the infrared radiation and converting the same into heat;        means for thermally insulating the detector, so as to allow it to warm up under the action of the infrared radiation;        thermometry means which, in the context of a bolometric detector, employ a resistive element;        and means for reading the electric variables supplied by the thermometry means.        
Detectors intended for infrared imaging are produced conventionally in the form of a matrix of elementary bolometric detectors, or bolometers, arranged according to one or two dimensions, the matrix being suspended above a substrate, generally made out of silicon, via support arms for each elementary detector.
Provision is commonly made in the substrate for means for the sequential addressing of the elementary detectors and means for the electrical excitation and pre-processing of the electrical signals generated by these elementary detectors. These sequential addressing, electrical excitation and pre-processing means are therefore formed in the substrate and constitute a read circuit.
To obtain the image of a scene through the medium of this detector, the scene is projected through an appropriate optic onto the matrix of elementary detectors, and timed electrical stimuli are applied through the medium of the read circuit to each of the elementary detectors, or to each row of the detectors, in order to obtain an electrical signal constituting the image of the temperature reached by each of the elementary detectors. This signal is processed in a more or less sophisticated way by the read circuit, and then possibly by an electronic device external to the enclosure in order to generate the thermal image of the scene observed.
Such a detector has many advantages in terms of manufacturing cost and use but also drawbacks which restrict its performance.
To be more specific, a problem is posed of the uniformity of the signal formation by the bolometer matrix. Indeed, owing to a dispersion in the behavior of the bolometers, which do not all produce the same output level when they are brought to one and the same operating temperature, and which do not all respond exactly in the same way in the face of one and the same infrared radiation coming from the scene, the image of a uniform scene formed by the detector has a fixed pattern noise.
The dispersion can have a great many causes. We may cite the main cause as being technological dispersion of the bolometers which translates into resistance dispersion, entailing for the detectors a variation in their output levels, even when the matrix is fed by a uniform scene. Another cause of signal dispersion is thermal drift in the substrate and in its spatial temperature distribution, given that bolometers keep to the temperature of the substrate on which they are supported.
Commonly, the shift in the output level of one specific bolometer relative to the average output level of the bolometers in the matrix when it is fed by a uniforms scene, is denoted by the term “offset”. The term “Continuous Level (Niveau Continu)”, or NC in the interests of simplification, is used hereinafter to denote all the output levels in these particular uniform feed conditions.
To compensate for bolometer offset dispersion, which is the main cause of limitation in the quality of the signals supplied by the detector, many correction techniques have been perfected.
One first type of offset correction, as described for example in document US 2002/022938, comprises acquiring a reference image, namely that of a uniform scene. This reference image is then stored in the system (a term used here to mean all the electromechanical and software functions implementing the detector or sensor), and then subtracted, digitally or analogically, from each image acquired thereby. The reference image is commonly farmed by means of an essentially isothermic shutter, which is closed in order to obtain the uniform scene.
This first technique has the drawback of rendering the detector inoperative throughout the entire reference image acquisition time. Furthermore, installing a shutter involves a not inconsiderable additional cost and increases the sources of mechanical failure in the system and the energy consumption thereof.
The second type of offset dispersion correction is based on the fact that the offset of a resistive bolometer depends on the temperature thereof. In this second type of bolometer correction, as described for example in the document U.S. Pat. No. 5 811 808, offset tables in respect of different predetermined temperatures of the sensor are stored permanently in the system. The system is provided with a thermometric sensor that measures the temperature of the substrate, and a data processing unit then selects one of the stored offset tables as a function of the measured temperature, or creates a new offset table by interpolating stored tables as a function of the measured temperature. The offset table selected or created is then subtracted from the current image detected.
This type of correction does not then need a shutter but does however prove to be less effective than the first correction type. Furthermore, the higher the number of reference points and the degree of the interpolating polynomial the greater the accuracy of the correction by interpolation. In fact, quality interpolation requires significant quantities of calculation resources, and storage of a sufficient number of tables. Moreover, the offset table acquisition time is significant. Lastly and above all, by virtue of its principle, an interpolation is only valid with accuracy in the vicinity of the reference points. The manufacturing cost of a detector implementing the second technique thus becomes prohibitive as soon as accurate sampling is required of the operating temperature range of the detector.
The purpose of the invention is to resolve the aforementioned problems, by proposing an effective and accurate correction technique, which does not require a shutter, while using a limited quantity of tables.